A stepped automatic transmission is composed of planetary gears and a plurality of friction elements (brakes, clutches), and a plurality of gear positions are realized by changing engaged states of the plurality of friction elements. The friction elements to be engaged are determined for each gear position. In the time of realizing a certain gear position, a shock or an interlock occurs if the friction element, which is not supposed to be engaged in this gear position, is engaged. Thus, it is desirable to drain a hydraulic pressure of the friction element, which is not supposed to be engaged, and prevent this friction element from being mistakenly engaged.
FIGS. 8A to 8C are schematic diagrams showing a supplied state of a hydraulic pressure to a B2 brake of an automatic transmission disclosed in JP2002-147587A.
The B2 brake is a friction element which is engaged only in a first gear position in an L position and in a reverse gear position in an R position. Since an interlock occurs if the B2 brake, which is not supposed to be engaged, is engaged at the time of selecting a D position, the B2 brake needs to be reliably released when selecting the D position. Thus, in JP2002-147587A, a B2 brake hydraulic pressure is drained via a sequence valve and a manual valve as shown in FIG. 8A when selecting the D position so that the B2 brake is not engaged even if a hydraulic pressure is output from a B2 pressure control valve.
In JP2002-147587A, if a selector is operated to the R position in a state of FIG. 8A, a solenoid is turned on during a shift associated with a position change (D-R position transient state) and a B2 brake pressure PB2 adjusted by the B2 pressure control value is supplied to the B2 brake as shown in FIG. 8B. This is for gradually engaging the B2 brake by gradually increasing the B2 brake pressure PB2 to alleviate a shock at the time of engagement. After the shift is completed (R position steady state), the solenoid is turned off again and an R position pressure PR (full hydraulic pressure) from the manual valve is supplied to the B2 brake as shown in FIG. 8C.